Generally, in air conditioning apparatus, thermal control is accomplished by intermittent operation of the compressor in response to a signal from a thermostat located in the room being cooled. Once the temperature in the room has been lowered to a desired temperature, the refrigerant capacity of the air conditioning system generally need not be very large in order to handle supplemental cooling because of further temperature changes in the room or to keep the room at the desired temperature. Accordingly, after the room has cooled down to the desired temperature, the most common technique for controlling the output of the compressor is by intermittent operation of the compressor. However, this intermittent operation of the compressor results in the intermittent application of a relatively large load to the driving mechanism of the compressor.
In automobile air conditioning compressors, the compressor is driven by the engine of the automobile through an electromagnetic clutch. Automobile air conditioning compressors face the same intermittent load problems described above once the passenger compartment reaches a desired temperature. Control of the compressor normally is accomplished by intermittent operation of the electromagnetic clutch which couples the automobile engine to the compressor. Thus, the relatively large load which is required to drive the compressor is intermittently applied to the automobile engine.
Furthermore, since the compressor of an automobile air conditioner is driven by the engine of the automobile, the rotation frequency of the drive mechanism changes from moment to moment, which causes the refrigerant capacity to change in proportion to the rotation frequency of the engine. Since the capacity of the evaporator and the condenser of the air conditioner does not change when the compressor is driven at high rotation frequency, the compressor performs useless work. To avoid performing useless work, prior art automobile air conditioning compressors often are controlled by intermittent operation of the magnetic clutch. However, this again results in a large load being intermittently applied to the automobile engine.
One solution to above mentioned problems is to control the capacity of the compressor in response to refrigeration requirements. One construction to adjust the capacity of a slant plate type compressor, particularly a wobble plate type compressor, is disclosed in the U.S. Pat. No. 3,861,829 issured to Roberts et al. Roberts et al. '829 discloses a wobble plate type compressor which has a cam rotor driving device to drive a plurality of pistons and varies the slant angle of a slant surface to change the stroke length of the pistons. Since the stroke length of the pistons within the cylinders is directly responsive to the slant angle of the slant surface, the displacement of the compressor is easily adjusted by varying the slant angle. Furthermore, variations in the slant angle can be effected by the pressure difference between a suction chamber and a crank chamber in which the driving device is located.
In these prior art compressors, the slant angle of the slant surface is controlled by pressure in the crank chamber. Typically this control occurs in the following manner. The crank chamber communicates with the suction chamber through an aperture and the opening and closing of the aperture is controlled by value mechanism. The value mechanism generally includes a bellows element and a needle valve, and is located in the suction chamber so that the bellows element operates in accordance with changes of pressure in the suction chamber. The acting point of valve mechanism at which it opens or closes the aperture is determined by the pressure of the gas contained in bellows element. The acting point of bellows element is thus fixed at a predetermined value. The bellows element therefore operates only at a certain change of the pressure in the suction chamber, and can not respond to various changes of refrigerating conditions since the bellows element is set to act at a single predetermined pressure. Furthermore, since the predetermined acting point of the bellows element can not be changed, the valve can not be made responsive to requirements such as when the air conditioner requires an especially low evaporating temperature or the compressor must operate with small volume for decreasing thermal loads. Also, for the purpose of reducing the number of parts in a compressor an electromagnetic clutch can be omitted and the compressor can be directly connected to a driving source. In this type of compressor, the compressor is driven whenever the driving source is operating. Operation of this type of compressor is especially difficult when the value of the predetermined operating point of bellows element can not be changed with changes in the thermal load of an evaporator in a refrigerant circuit.
Roberts et al. '829 discloses the capacity adjusting mechanism used in a wobble plate type compressor. As is typical in this type of compressor, the wobble plate is disposed at a slant or incline angle relative to the drive axis, nutates but does not rotate, and drivingly couples the pistons to the drive source. This type of capacity adjusting mechanism, using selective fluid communication between the crank chamber and the suction chamber, however, can be used in any type of compressor which uses a slanted plate or surface in the drive mechanism. For example, U.S. Pat. No. 4,664,604, issued to Terauchi, discloses this type of capacity adjusting mechanism in a swash plate type compressor. The swash plate, like the wobble plate, is disposed at a slant angle and drivingly couples the pistons to the drive source. However, while the wobble plate only nutates, the swash plate both nutates and rotates. The term slant plate type compressor will therefore be used herein to refer to any type of compressor, including wobble and swash plate types, which use a slanted plate or surface in the drive mechanism.